1. Field of Invention
This invention relates to method and apparatus for optically measuring the pattern areas which are necessary for presetting an ink supply in offset printing from an engraving film.
2. Description of the Prior Art
In offset printing, it is necessary that the ink supply be different for each of a number of columns which are created by widthwise division along the printing direction.
For example, as shown in FIG. 1A more specifically, the ink in an ink reservoir IK is supplied through a reservoir roller RL1, a transfer roller RL2, averaging rollers RL3, kneading rollers RL4 and ink applying rollers RL5 to a printing plate RL7 which is attached to the outer circumference of a printing drum RL6.
This supply is controlled such that the gap between a doctor blade DCj and the roller RL1 is adjusted by turning a screw SCRj. Several doctor blades DCj and screws SCRWj are provided in the axial direction of the roller RL1 for respective columns CP(i) (as shown in FIG. 18).
As described above, the ink supply device for offset printing is usually constructed such that the ink from the ink reservoir passes through a roller train composed of the plurality of rollers until a sheet of ink having a uniform thickness is supplied onto the printing plate. Moreover, the ink is applied only to the pattern portion, so the ink corresponding to nonpattern portions is left on the roller train.
It is, therefore, necessary that the ink be supplied at a rate corresponding to the consumption rate from the ink reservoir.
It is usual that the consumption rate of the ink is proportional to the area of the pattern portion (i.e., the pattern area). Since the areas of non-pattern portions are different in the widthwise direction of the printed matter, it is, therefore, necessary in offset printing that the ink supply be adjusted in the widthwise direction of the printed matter. For this necessity, the printing surface is widthwise divided into columns having a suitable width (e.g. 30 mm), for which the ink supply is respectively adjusted.
In order to improve the productivity of the printing process and to reduce inferior printing, it is necessary that the aforementioned ink supplies for the respective columns be preset before printing begins. In order to get data for that presetting operation, moreover, it is necessary that the area of the pattern portions or the non-pattern portions be measured for the aforementioned reasons. For measurement of the pattern areas, there has already been known in the art a system in which the images corresponding to the images to be printed are optically scanned to calculate the areas of the image portions, respectively, for the adjusting columns. The image holders to be scanned may be a proof, a printing plate or an engraving film.
The former two are measured by a reflection detecting system, whereas the engraving film can be measured by a light transmissive detecting system.
In case the pattern area is to be optically measured, the measurement of the engraving film is advantageous. However, the usual printing plate is so constructed that a plurality of films are arranged in predetermined positions and printed. In case the films are to be measured, therefore, it is necessary that, after the pattern areas of the respective films have been measured, the measured data be summed up again in a manner to correspond to the positions to which the films are assigned. This task become more difficult as the layout of the printing plate becomes more complex.